Working cycle for dispersion strengthened materials



Jan.30,1968 R WFRASER ETAL 3,366,515

WORKING CYCLE FOR DISPERSION STRENGTHENED MATERIALS Filed June 21, 1965 F/ G- 7 PR/07/7PAR7 F/ G- 3 PR/Of? ART Inventors ROBERT W FRASER DAVID J. EVANS United States Patent Oil 3,366,515 Patented .lan. 3t), 1968 ice 3,366,515 WORKING CYCLE FOR DISPERSION STRENGTHENED MATERIALS Robert William Fraser, Fort Saskatchewan, Alberta, and David John Ivor Evans, North Edmonton, Alberta, Canada, assignors to Sherritt Gordon Mines Limited, Toronto, Ontario, Canada, a company of Ontario, Canada Filed June 21, 1965, Ser. No. 465,291 Claims priority, applicafingcganada, Mar. 19, 1965, 5 13 Claims. (Ci. 148-115) This invention relates to a method for working substantially fully dense metallic materials containing dispersion strengthening refractory oxide particles. More particularly, it is concerned with a cyclic cold working-annealing process for working dispersion strengthened metal and alloy compositions to produce wrought products which exhibit improved high temperature strength characteristics. It is also concerned with the products of such process.

Cold working and annealing are techniques which are well known and in constant use in the metallurgy art. Cold working is the term used to encompass the methods of plastically deforming a metal or alloy by the action of applied stresses at a temperature below the recrystallization temperature of the material. Generally, cold working metals and alloys results in an increase in their strength and hardness and a decrease in their ductility. Coldworked metals and alloys can be softened or annealed by heat treatment.

The eliects of cold working and annealing have been explained as follows: Cold working increases the number of defects, known as dislocations, within the atomic lattice of the worked metal. The stresses which are applied during working cause the dislocations, both structural and those generated during working, to move through the atomic lattice of the metal resulting in plastic deformation. This movement of dislocations is hindered by a number of obstacles including grain boundaries and other dislocations. An increase in the number of obstacles to dislocation movement, such as an increase in the dislocation density or the altering of a coarse grained metal to a fine grained metal, will result in an increase in resistance to plastic deformation. The strength characteristics of a metal will therefore increase and the ductility decrease with an increase in its dislocation content and a decrease in its grain size.

Annealing a cold-worked metal or alloy will result in an increase in the ductility of the material and a decrease in strength and hardness gained by cold Working. It has been postulated that the addition of thermal energy during the annealing process causes migration and self-annihilation of dislocations to occur whereby there is a decrease in number and a re-arrangement of dislocations. These effects normally are accompanied by the nucleation and growth of new grains with a low dislocation content, a mechanism known as re-crystallization and grain growth. It has been suggested that new grain nucleation takes place in areas within the grain and at the grain boundaries where there are concentrations of dislocations. A metal which has undergone substantial plastic deformation by cold working and which consequently contains many dislocations therefore has more sites conducive to nucleation than a metal with a comparatively low dislocation content. When a high dislocaton content metal is annealed at temperatures above the recrystallization temperature, many new grains are nucleated and commence growing; however, they restrict each others growth and the resulting product is time grained in comparison to the product which results when low dislocation content material is similarly annealed. Thus, the cold working techniques which are conventionally employed on metals and alloys to improve strength characteristics are those wherein relatively large reductions in cross-sectional area, i.e., 20% or more, are taken in order to generate a high dislocation content and thereby to promote a fine grained material on annealing. Heretofore, similar cold working techniques have been applied to dispersion strengthened materials comprised of a dispersion of ultra-fine refractory oxide particles in a metal or alloy matrix. However, although dispersion strengthened wrought products possessing improved high temperature strength and other desirable properties as compared to unmodified metals and alloys have been fabricated by these conventional working techniques, it is nevertheless highly desirable to obtain even more improved properties in such materials in order to meet the service requirements demanded by modern technology.

It is therefore an object of this invention to provide an improved method of working fully dense metals and alloys containing one or more dispersion strengthening refractory oxides which results in wrought products with improved high temperature strength characteristics.

It is a further object of the invention to provide a technique of cold working and annealing metals and alloys containing one or more dispersion strengthened refractory oxides which results in wrought products with a high dislocation density with the dislocation arrayed in a poly onized substructure.

Another object is to provide dispersion strengthened metals and alloys having improved high temperature strength characteristics and which are characterized by having relatively small grain size, polygonized substructure, relatively little evidence of re-crystallization and good dispersion of the refractory oxides in the matrix as compared to similar materials fabricated by conventional cold work-anneal treatment.

Broadly, the present invention by which these and other objects are achieved comprises a method of working substantially fully dense materials comprised of a metal or alloy containing a dispersion of ultra-fine refractory oxides by subjecting said material to a series of cold work-anneal cycles in which cross-sectional area reductions of less than 20%, and preferably less than 10%, are taken followed by controlled annealing after each such reduction.

The products of this method are wrought, dispersion strengthened compositions consisting essentially of a metal matrix containing a dispersion of sub-micron refractory oxide particles, said matrix having fibrous grains which are characterized by a polygonized substructure.

The invention will be described in detail hereinbelow in conjunction with the drawings in which:

FIGURE 1 is a drawing prepared from a photomicrograph of a section of a specimen of 97.5 nickel-2.5 thoria (wt. percent) dispersion strengthened material Worked in accordance with the prior art methods, magnification 600 X;

FIGURE 2 is a drawing prepared from a photomicrograph of a section of a specimen of 97.5 nickel-2.5 thoria (wt. percent) dispersion strengthened material worked in accordance with the method of the invention, magnification 600 X;

FIGURE 3 is a drawing of an electron micrograph of a small area of the prior art material depicted in FIC- URE 1, illustrating the substructure of the material, magnification 30,000 X;

FIGURE 4 is a drawing of an electron micrograph of a small area of the material of FIGURE 2 illustrating the substructure which is characteristic of materials of the present invention, magnification 30,000 X.

In practice, the method of this invention is applied to a substantially fully dense composition comprised of a dispersion of sub-micron refractory oxides in a metal or alloy matrix. The invention is not concerned with the precise manner in which such dense composition is prepared. It is known that powder metallurgy composition of one or more metals and one or more refractory oxides may be formed into a partially densified, self-supporting green compact which may then be hot worked to give a substantially 100% dense article which can be further worked to provide a wrought, dispersion strengthened product of desired size and shape. Such powder compositions may consist of a physical mixture of fine metal powders and ultra-fine refractory oxide particles. Preferably, however, the refractory oxide component of the composition is incorporated in or integrally associated with one or more of the metal powder constituents. These latter compositions are generally preferred to simple physical mixtures of oxides and metal constituents for fabrication of dispersion strengthened products since the problems of segregation and agglomeration of the oxide particles are largely overcome and, at the same time, it is generally possible to obtain a more uniform distribution of the oxide particles in the matrix of the densified product.

In the practice of the present invention, a substantially 100% dense metal or alloy-refractory oxide composition obtained, for example, by hot working a green compact derived from conventional static or roll compacting operations is first cold-worked to effect a reduction in crosssectional area of less than 20% and preferably less than Any conventional mechanical working methods may be used including, for example, rolling, swaging, forging, drawing and extrusion. The required reductions may be taken in one or more separate working operations, the only controlling factor being that the total reduction between anneals must be less than 20%.

The cold worked material is then annealed. The precise annealing temperature will, of course, be determined by the nature and composition of the particular material being treated. However, a number of general considerations will always apply. The annealing must be conducted at a temperature which is sufficiently high to ensure that residual strains are relieved in a reasonably short time yet sufiiciently below the melting point of the matrix to prevent agglomeration of the dispersed oxides phase. In most cases, the annealing temperature will be above the re-crystallization temperature of the matrix. However, in some metals and alloys, strain relief can be obtained by heating the material below the re-crystallization temperature.

The duration of the annealing step is not critical. In the case of conventionally worked materials, excessive anneal times result in a large amount of re-crystallization and must therefore be avoided. However, with the method of this invention, because of the fewer dislocations created in any one working step, there is very little tendency for new grains to nucleate and grow even with prolonged annealing.

Normally, annealing is carried out in a reducing atmosphere to prevent the formation of oxides and to help remove oxides present in the composition other than the dispersed refractory oxides. Inert atmospheres may also be employed where there is no need to remove contaminating oxides from the compositions.

Optimum anneal conditions for any specific material will be readily determinable by those skilled in the art by simple experiment and having regard to the teachings herein.

The cold work-anneal cycle is repeated as often as is required to make a wrought product of the desired shape and dimensions.

Wrought products worked by the method of this invention possess improved high temperature strength characteristics as compared to products of the same composition worked by conventional methods. Further, the products have a novel structure to which, it is believed, the improved strength characteristics can be attributed. This novel structure can best be explained by reference to the drawings which are based on micrographic studies of specimens of wrought dispersion strengthened nickel worked by conventional methods and by the method of this invention. FIGURE 1 depicts the grain structure of a specimen of dispersion strengthened nickel containing 2.3 volume percent thoria as the dispersoid and worked in accordance with the prior art. The specimen was obtained by reducing a substantially dense strip of the material from an initial thickness of 0.10 inch to a final thickness of 0.01 inch by taking six cold rolling reductions of about 30% each and following each reduction with a 30%-minute anneal at 2200 F. The drawing shows the relatively large grains 10 of this material and the presence of annealing twins as indicated by the numeral 11.

FIGURE 2 depicts the grain structure of a specimen of the same fully dense material as that of FIGURE 1 but worked in accordance with the instant invention. Specitically, the dense material was reduced from 0.10 inch thickness to 0.01 inch thickness by taking 21 reductions of about 10% each followed by 30-minute anneals at 2200 F. It will be noted that the grains 12 of the material are smaller and less equi-axed than the grains of the material shown in FIGURE 1 which was subjected to a 30-minute anneal at 2200 F. The drawing shows the These characteristics indicate substantially less re-crystallization in the material of FIGURE 1 than in the material of FIGURE 2.

FIGURE 3 and FIGURE 4 are based on electron mrcrographs of the specimens of FIGURES 1 and 2, respectively. Magnification is 30,000 and the areas enlarged are indicated by the squares A and B in FIGURES 1 and 2. FIGURE 3 shows the substructure of the prior art material which is characterized by high-angle sub-grain boundaries 13 and partial re-crystallization evidenced by large annealing twins 14 and dense tangles of dislocations 15. The material of this invention depicted in FIGURE 4 has a substructure known as a polygonized substructure which is characterized by low-angle sub-grain boundaries 16, and a lack of evidence of re-crystallization. Dark spots 17 are thoria particles randomly dispersed in the nickel material of each of the specimens.

The method of this invention is applicable to any dispersion strengthened metals and alloys which display useful high temperature strength properties when fabricated by conventional methods. The method is particularly useful in working compositions in which the matrix is comprised of iron, nickel, cobalt and copper as well as alloys based on these metals and the dispersed refractory oxide particles are formed of Y O T'hO MgO, CaO, Zl'Og, SiO U02, 1.3203, BeO, A1203, Hfoz, C60 or or mixtures of two or more of these oxides. Compositions in which the matrix is comprised in whole or in part of other metals including beryllium, magnesium, aluminum, manganese, chromium, molybdenum, niobium, tantalum, titanium, vanadium, uranium, zirconium, platinum, palladium, gold and lead may also be worked in accordance with this invention to improve their strength properties at elevated temperatures below the melting point of the metal or alloy. The refractory oxide particles should be uniformly dispersed in the matrix and should be less than 100 millimicrons in size and preferably in the size range of 10 to 30 millimicrons. The amount of refractory oxide can vary from a trace to 30% by volume or more depending on the type of oxide employed and the properties desired in the end product, and normally will be in the range of about 2.04.0% by volume.

The invention is based on observations of actual physical results; however, it is believed that these results may be explained theoretically as follows: Taking smaller reductions, i.e., less than 20%, during a working cycle reduces the number of dislocations created in the material per cycle. We believe that this reduction in dislocation density enables the dislocations to move more freely through a grain, thus making it easier for the dislocations to array themselves in the polygonized substructure during annealing. Also, with a reduced number of areas of strain and dislocation concentration, there is substantially less recrystallization during annealing than that which occurs with more severely worked materials. Therefore, dislocations formed during the small reductions remain locked within the polygonized structure rather than being annihilated by re-crystallization. As the material is worked with small reductions and annealed repeatedly in cycles, the dislocation content is gradually increased to a high level while re-crystallization is kept to a minimum. FIG- URES 3 and 4 clearly illustrate the difference in the degree of re-crystallization in materials worked by a conventional method and those worked by the method of this invention.

The dispersed particles of refractory oxides have the effect of helping to lock and stabilize the dislocations within the low-angle grain boundaries or polygonized structure by providing obstacles which prevent the movement of the dislocations which normally tends to occur when the material is heated.

The following examples, describing the fabrication of wrought nickel-thoria and nickel chromium-thoria-magnesia strip and comparing the high temperature service characteristics obtained by varying the percentages in reduction of cross-sectional area in the working of the strip will serve to illustrate the invention.

Example I Nickel-thoria powder was compacted in a 1.25 inch by 2.4 inch die under a IOO-ton load to make a 60 gram billet 0.20 inch in thickness. The nickel-thoria powder was obtained by the hydrometallurgical process described in co-pending United States application Ser. No. 543,495, filed Apr. 18, 1966, and had the following characteristics.

The billet which was about 60% fully dense, was heated in a flowing hydrogen atmosphere to a temperature of 2200" F. to prepare it for hot working and to remove any nickel oxide present. It was then hot rolled to take a 50 percent reduction, resulting in a substantially 100% dense strip product 0.10 inch in thickness. The strip was subsequently annealed in a flowing hydrogen atmosphere at 2200 F. for 30 minutes.

The fully dense strip was cooled and cold rolled to take a reduction. Following cold working, the strip was annealed at 2200 F. for 30 minutes and the cold rollanneal working cycle was repeated with 10% reductions until the strip had undergone a total of 21 work-anneal cycles resulting in a final strip thickness of 0.01 inch.

The ultimate tensile strength of the fully dense strip at elevated temperatures was increased as follows:

Pounds per square inch Ultimate tensile strength of hot rolled strip at 1600 F. Ultimate tensile strength of cold worked strip at Example 11 Ultimate tensile strength of cold worked strip at 1600 F.23,000 p.s.i.

6 It can be noted that the high temperature tensile strength of material worked by the method of this invention (Example I) was about 23% higher than that obtained by conventional working (Example H).

Example III Nickel-thoria, chromium and magnesium powders were mechanically blended to provide a powder composition containing: thoria 2.5 percent by weight, magnesium 0.5 percent by weight, chromium 20 percent by weight and the balance nickel. The chromium and magnesium were commercial grade powders sized to pass a 325 mesh Standard Tyler screen. The nickel-thoria powder was the same as that employed in Examples I and H.

The composition was compacted into a 30-gram billet, 0.10 inch in thickness, and densified by hot working to give a fully dense strip 0.05 inch in thickness. The strip was then subjected to 8 working cycles of 10 percent cross-sectional area reduction followed with 30-minute anneals at 2200" F. after each reduction to produce a strip .020 inch thick. The ultimate tensile strength of the cold worked strip at 1600 F. was 23,700 p.s.i.

Example 1V For comparison purposes, a billet of a composition similar to that of Example III was worked in the same manner as that of Example IH except that 3 reductions in cross-sectional area of 40 percent each were taken. The ultimate tensile strength of the cold worked material at 1600 F. was 16,100 p.s.i.

It will be understood, of course, that modifications of the preferred embodiment of the invention described herein can be made without departing from the scope of the invention defined by the appended claims.

What we claim as new and desire to protect by Letters Patent of the United States is:

1. A process for producing wrought dispersion strengthened metal and alloy compositions having improved high temperature strength characteristics which comprises providing a substantially fully dense shape comprising a metallic matrix containing dispersed sub-micron refractory oxide particles, said matrix being formed of at least one metal selected from the group consisting of iron, nickel, cobalt, copper, beryllium, magnesium, aluminum, manganese, chromium, molybdenum, niobium, tantalum, titanium, vanadium, uranium, zirconium, platinum, palladium, gold and lead; cold working said shape to effect a cross-sectional area reduction thereof of less than about 20%; annealing said cold worked shape at a temperature sufiicient to relieve residual strains but below the melting point of the matrix; and repeating said cold working and annealing steps to gradually increase the dislocation content of the matrix while keeping recrystallization and grain growth to a minimum.

2. The method according to claim 1 wherein about a 10% reduction in cross-sectional area is effected in each cold working cycle.

3. The method according to claim 2 wherein the matrix is formed of nickel.

4. The method according to claim 2 wherein the refractory oxide is thoria.

5. The method according to claim 2 wherein the matrix is an alloy of nickel and chromium.

6. The method according to claim 2 wherein the matrix is nickel and the refractory oxide is thoria having a particle size within the range of 5 to 30 millimicrons.

7. The method according to claim 6 wherein the substantially fully dense shape is in the form of strip, the cold working is effected by rolling and each annealing step is conducted at a temperature of about 2100 F.

8. A cold worked and annealed dispersion strengthened composition consisting essentially of a dispersion of sub-micron refractory oxide particles in a matrix formed of at least one metal selected from the group consisting of nickel, cobalt, iron, chromium, copper, molybdenum,

niobium, tantalum, titanium, vanadium, uranium, tungstem and zirconium; said matrix having fibrous grains with a polygonized substructure characterized by low angle sub-grain boundaries and substantially no recrystallization.

9. The composition of claim 8 wherein the matrix is formed of nickel.

10. The composition of claim 9 wherein the refractory oxide particles are formed of thoria.

11. The composition of claim 8 wherein the matrix is formed of an alloy of nickel and chromium.

12. The composition of claim 11 wherein the refractory oxide particles are formed of thoria and magnesia.

References Cited UNITED STATES PATENTS 3,159,908 12/1964 Anders 29-1825 DAVID 'L. RECK, Primary Examiner. H. SAITO, W. W. STALLARD, Assistant Examiners. 

1. A PROCESS FOR PRODUCING WROUGHT DISPERSION STRENGTHENED METAL AND ALLOY COMPOSITIONS HAVING IMPROVED HIGH TEMPERATURE STRENGTH CHARACTERISTICS WHICH COMPRISES PROVIDING A SUBSTANTIALLY FULLY DENSE SHAPE CONPRISING A METALLIC MATRIX CONTAINING DISPERSED SUB-MICRON REFRACTORY OXIDE PARTICLES, SAID MATRIX BEING FORMED OF AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF IRON, NICKEL, COBALT, COPPER, BERYLLIUM, MAGNESUM, ALUMINUM, MANGANESE, CHROMIUM, MOLYBDENUM, NIOBIUM, TANTALUM, TITANIUM, VANADIUM, URANIUM, ZIRCONIUM, PLATINUM, PALLADIUM, GOLD AND LEAD; COLD WORKING SAID SHAPE TO EFFECT A CROSS-SECTIONAL AREA REDUCTION THEREOF OF LESS THAN ABOUT 20%: ANNEALING SAID COLD WORKED SHAPE AT A TEMPERATURE SUFFICIENT TO RELIEVE RESIDUAL STRAINS BUT BELOW THE MELTING POINT OF THE MATRIX; AND REPEATING SAID COLD WORKING AND ANNEALING STEPS TO GRADUALLY INCREASE THE DISLOCATION CONTENT OF THE MATRIX WHILE KEEPING RECRYSTALLIZATION AND GRAIN GROWTH TO A MINIMUM. 